Unfortunately, some porous cloth does not show tiny holes when held up to
the light.  Not only that, some cloth looks fine and is tight, for a month
or so after application, then begins to leak!!!    I assume that this is
due to the cloth being stretched over the boards, thus straining the
low-grade rubber to the point of microscopic breakage.  This is
particularly exasperating, although usually the defect can be found right
away.

The only reliable test I know is to glue the material to an airtight
structure, then test under pressure.  A bellows or pneumatic should be
simply impossible to collapse without damage, if its outlet is plugged.  If
it slowly or rapidly closes, something is wrong.

This reminds me of another subtlety in the world of airtightness:

When doing museum work several years ago on a "carte blanche" basis, in the
pursuit of perfection I decided to perform suction tests on the shellac
membrane with which manufacturers sealed wood in the old days; for example,
valve wells, manifolds, etc.  In almost all cases, I found it highly
porous!

It quickly became apparent that in many cases, all manifolds and borings in
wooden chests needed to be filled to the top with shellac, then drained, as
most player action manufacturers did.  (I disliked using other sealants,
for reasons of authenticity and -- because I had seen how previous
applications of lacquer, etc., by other restorers, simply sat on the
surface of the old shellac and did not bond to it.)

I first made a test block, by drilling a large hole in wood and covering it
with a carefully glued pump-cloth patch.  I then applied mouth suction to
an outlet I had provided, through a rubber tube.  Compared to the suction I
could achieve by pinching the tube, that achievable in communication with
the wood block was negligible.

Next, using a can of shellac (3 lb. cut) from the store, I filled and
drained the chamber in my wood block, and allowed it to dry thoroughly.
Compared to the pinched tube, it still was not airtight!  Finally, a third
application of shellac rendered it "fairly" airtight.

I was very displeased with these results, so I contacted a chemist at the
William Zinsser Co., makers and suppliers of shellac.  He urged me to make
my own shellac from flakes (which they sell) and denatured alcohol.

After doing so I repeated my test.  A single application of this homemade
shellac produced 100% airtightness.  I remember showing this to another
well-known restorer at the time, who was amazed at the difference.

The reason is that shellac at stores is usually simply too old to produce
optimal results.  It must be made reasonably fresh.

I have chambers in wood I sealed in this way in 1980, and the shellac looks
as good and is as airtight as it was then.

Although it obviously does not last forever, I still believe that, for most
restoration work, flake shellac, preferably orange (for the outside of most
striking pneumatic boards I use white shellac, identical to that used on
most originally), is the ideal sealant for wood. It marries perfectly to
original shellac and remelts it slightly.  It is bound to have a
satisfactory life span (outlasting cloth and leather, etc.) and that is
good enough.  Shellac remains slightly elastic for many years, thus it will
not crack away from its moorings.  It also sticks well to metal.  (For new
work, I see no reason why other, modern sealants would not work as well or
better).

Shellac is highly unsatisfactory as an adhesive or filler unless specially
prepared.  Just burning off the alcohol does not make a good filler or
adhesive product.  If all surfaces are clean, high quality shellac can be
used thick, as with "pad cement" used on woodwind instruments.  For certain
player work, such as cementing in brass or nickel-plated brass nipples, I
recommend an amber-colored product called "3M-847," which is flexible,
sandable, quick setting and looks and behaves precisely like thick shellac,
only better.  I have used this for over a decade and therefore have some
experience with its behavior over time, etc.

Larry Broadmoore